1. Field of the Invention
The present invention relates to a capacitive type pressure sensing cell having deflecting plates or diaphragms useful for measuring fluid pressures acting on the plates and using brittle materials for the deflecting plates.
2. Description of the Prior Art
Various pressure sensing devices have been advanced in the art which utilize deflecting diaphragms, the movement of which is detected by capacitive sensing, and which include overpressure protection. Further, various pressure sensors, particularly differential pressure sensors, have utilized a fluid fill between the diaphragms which are deflecting.
Pressure sensing cells, which have sensing diaphragms on opposite sides of a center block, so that each of the diaphragms forms a separate chamber, with the chambers fluidly connected and wherein differentials in pressure on the respective diaphragms will cause deflection of such diaphragms are shown in U.S. Pat. No. 4,398,194, and also in U.S. Pat. No. 4,169,389. Each of these patents teach that differences in temperature will cause different volumes of oil filling the space enclosed by the diaphragms. In U.S. Pat. No. 4,398,194 this difference in volume is compensated by the sensing circuitry, while in U.S. Pat. No. 4,169,389 the differences in volume are compensated for mechanically. In each of these patents a center, non-deflecting mounting block is utilized, as distinguished from the present device where chambers are formed from two plates, both of which will be deflected by external pressure variations.
European patent publication No. 0111348 also shows a capacitance sensor having a central block with deflecting diaphragm on opposite sides of the block and a fluid filling under the deflector diaphragm.
U.S. Pat. No. 4,177,496 discloses a capacitive pressure sensor comprising two discs made from a brittle insulator material, such as alumina, fused silica or glass, and which have spaced surfaces, facing each other, with conductive layers on such surfaces. A single chamber is formed, and when the discs are subjected to external pressures the discs tend to move toward each other. The deflection of the discs is measured capacitively to provide an indication of the pressure acting on the discs. Portions of the discs are made to come into contact under overpressure to attempt to prevent damage to the sensor itself from excessive overpressures. A device similar to the one shown in U.S. Pat. No. 4,177,496 is also shown in U.S. Pat. No. 4,207,604.
U.S. Pat. No. 4,058,788 shows diaphragm assemblies each comprising a pair of flexible corrugated diaphragms sealed at their peripheries to form fluid filled chambers. The pressure being sensed acts on the outer surfaces of both of the diaphragms forming the respective chambers. Differential pressures acting on the diaphragms forming two fluidly connected chambers cause a movement which is sensed by a cantilever beam having strain gages for sensing the motion. The diaphragms are flexible, and metallic, and are not made of a brittle material.
A capacitor sensor shown in U.S. Pat. No. 4,168,518 is made of a material such as quartz and comprises two members that are joined together to form an interior chamber and which have portions that deflect toward each other in response to pressure acting on the exterior surfaces of the deflecting members.
U.S. Pat. No. 4,084,438 also shows a pressure sensing device which has spaced apart walls that are sealed at their peripheries to form an interior chamber, and capacitive sensing means on the inner surfaces of the walls, so that as the walls deflect under pressure and the spacing changes, an electrical signal is provided. The device shown in U.S. Pat. No. 4,084,438 further teaches the use of a material such as quartz for forming the sensor.
U.S. Pat. No. 4,332,000 is of interest in that it shows capacitive pressure sensors formed from semiconductor material and discloses forming the sensor and the sensing circuity in the same process.
U.S. Pat. No. 4,390,925 also deals with a multiple cavity, pressure sensor formed on a silicon wafer.
U.S. Pat. No. 4,244,228 shows a pressure sensor which comprises slightly cupped shaped disc members that are joined at their peripheral edges and which will deflect under pressure. Under excessive pressures the plates will rest against each other. The discs are formed in a cup shape and made of a spring material in order to operate.
U.S. Pat. No. 4,301,492 shows a pressure sensor which utilizes diaphragms that are formed in a stacked arrangement, filled with fluid, and which will deflect under pressure. This is sensed by capacitive sensors, and is compensated for errors due to temperature shifts. The chambers are formed by having the deflecting diaphragms spaced from each other by annular rims in a rest position. The sensor can be used for measuring differential pressures, but does require, in the form disclosed, recesses formed in the diaphragm discs to provide the necessary spacing.
U.S. Pat. No. 4,163,395 shows a sensor for sensing differential pressures that has flat diaphragms, the space between which is oil filled to set the diaphragm spacing. The diaphragms will "bottom out" under overload. An external sensor is used, apart from the deflecting diaphragm assembly, and the oil from the space between the diaphragms acts on the external sensor as pressure changes.
The prior art thus does teach use of brittle materials for forming deflecting members in pressure sensors. Brittle materials are desirable because of low hysteresis and repeatability. Overpressure protection is also necessary, and the sensor construction of the present invention provides simple and effective overpressure protection in both directions, while also providing a sensor which is low cost and accurate and may be made in a batch process.